1. Field of the Invention
The present invention relates to a light emitting device using a phosphor or a light emitting element and a method of manufacturing the same. More particularly, the present invention relates to a light emitting device manufactured using an organic compound for a phosphor or a light emitting element and a method of manufacturing the same. Note that light emission in this specification includes fluorescence and phosphorescence and that the present invention includes light emission by any one of them or both.
2. Description of the Related Art
In a typical display device using liquid crystal, a back light is used and it is constructed such that an image is displayed by the back light. Although a liquid crystal display device is used as an image display means in various electronic devices, it has a structural defect such as a narrow view angle. On the other hand, since a display device using a phosphor for a pixel portion has a wide view angle and superior visibility, it is noted as a next generation display device.
A light emitting element using an organic compound for a phosphor (hereinafter referred to as an organic light emitting element) has a structure such as an appropriate combination of a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and the like, which are made of the organic compound, is provided between an anode and a cathode. Here, although the hole injection layer and the hole transport layer are separately indicated, these are identical in a sense that hole transportability (hole mobility) is a particularly important characteristic. For convenience, the hole injection layer is a layer in the side which is in contact with the anode. A layer in the side which is in contact with the light emitting layer is separately called the hole transport layer. A layer which is in contact with the cathode is called the electron injection layer and a layer in the side which is in contact with the light emitting layer is called the electron transport layer. There is a case where the light emitting layer also serves as the electron transport layer and thus it is called a light emitting electron transport layer. A light emitting element composed of a combination of these layers indicates a rectifying characteristic and thus it is considered that such light emitting element is one of diodes. In this specification, these are generically called an organic compound layer.
Both a small molecular system organic compound and a polymer system organic compound are known as organic compounds for forming the organic light emitting element. With respect to an example of the small molecular system organic compound, 4,4xe2x80x2-bis[N-(naphthyl)-N-phenyl-amino]-biphenyl (hereinafter referred to as xe2x80x9cxcex1-NPDxe2x80x9d) and 4,4xe2x80x2,4xe2x80x3-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (hereinafter referred to as xe2x80x9cMTDATAxe2x80x9d) as copper phthalocyanine (CuPc) aromatic amine system materials are known for the hole injection layer and tris-8-quinolinolato aluminum complex (Alq3) and the like are known for the light emitting layer. With respect to the polymer system organic compound, polyaniline, polyethylenedioxythiophene (PEDOT) as a polythiophene derivative, and the like are known.
In view of the variety of material, it is said that the small molecular system organic compound produced by an evaporation method has remarkable variety as compared with the polymer system organic compound. However, in any case, an organic compound purely made of only a base constitutional unit is rare. Thus, as a result of different kinds, there is the case where an impurity is mixed during a manufacturing process or various additives such as pigments are added. Also, these materials include a material deteriorated by moisture and a material which is easy to oxidize. Since mixing of moisture, oxygen, and the like from an atmosphere is easily allowed, it requires careful handling.
The light emitting mechanism is considered as a phenomenon such that an electron injected from a cathode and a hole injected from an anode are recombined in a light emitting layer made from a phosphor to form a molecular exciton and the molecular exciton emits light when it is returned to a ground state. As an excitation state, there are light emitting (fluorescence) from a singlet excitation state and light emission (phosphorescence) from a triplet excitation state. Since the intensity reaches several thousands to several tens of thousands of cd/m2, it is considered that this light emission mechanism can be applied, in principle, to a display device and the like.
On the other hand, there are various deterioration phenomena with respect to the organic light emitting element and thus these are regarded as problems. In particular, when the organic light emitting element is driven for a long time, there is a deterioration phenomenon such that a light emitting intensity decreases with the passage of time. Although this deterioration phenomenon is dependent on a drive condition such as a voltage applied to the organic light emitting device, the time when the light emitting intensity reaches a half of an initial value (a half life) is about 500 to 5000 hours. Thus, this becomes a large hindrance to practical use.
As one of reasons for deterioration of the organic light emitting element, it is known that the deterioration is progressed by only exposing it to air. One of reasons of such deterioration is considered that an alkali metal material composing a cathode reacts with moisture or oxygen. Thus, the organic light emitting element is sealed in a closed space and further the closed space is filled with a drying agent to take a measure for minimizing the deterioration.
However, even if such a sealing structure is used, the deterioration of the organic light emitting element cannot be completely prevented under the current state. In view of such a condition, it can be expected that the deterioration of the organic light emitting element is progressed even with a trace of moisture or oxygen present. Also, it can be considered that some factor other than such a factor is present.
With respect to an oxygen molecular, a highest occupied level of a molecular orbit (HOMO) is degenerated, and thus it is a specific molecular having a triplet state in a ground level. Generally, an excitation process from a triplet to a singlet is a forbidden transition (spin forbidden transition) and thus is hard to produce. Therefore, the oxygen molecular having the singlet state is not generated. However, if a molecular having the triplet excitation state (3M*) which is a higher energy state than the singlet state is present around the oxygen molecular, the following energy transfer is produced and thus a reaction whereby the oxygen molecular having the singlet state is generated can be induced.
3M*+3O2xe2x86x92M+1O2xe2x80x83xe2x80x83Formula 1
It is said that 75% of excitation states of molecular in the light emitting layer of the organic light emitting element are the triplet state. Therefore, when the oxygen molecular is mixed into the organic light emitting element, generation of the oxygen molecular having a singlet state is allowed by the energy transfer as indicated in the reaction formula (1). Since the oxygen molecular having the single excitation state has an ionic (charges are unbalanced) characteristic, a possibility that the molecular reacts to the unbalance of charges generated in the organic compound is considered.
For example, with respect to bathocuproine (hereinafter referred to as xe2x80x9cBCPxe2x80x9d), since a methyl group has an electron donating characteristic, carbon directly bonded to a conjugated ring is positively charged. As indicated by the following structural formula (I), when a positively charged carbon molecular is present, there is a possibility that singlet oxygen having an ionic characteristic reacts to it and carboxylic acid and hydrogen can be produced as indicated by the following structural formula (II). As a result, it is expected that an electron transport characteristic is deteriorated. 
Of course, such a change in a bonding state is one example of consideration in which the phenomenon is simplified. However, it can be explained that impurities such as oxygen and moisture, which are included in the organic compound, cause various deterioration phenomena such as intensity reduction.
An application example using the organic light emitting element is an active matrix drive light emitting device in which a pixel portion is composed of the organic light emitting element. The active matrix drive light emitting device in which the pixel portion is composed of a combination of the organic light emitting element and a thin film transistor (hereinafter referred to as xe2x80x9ca TFTxe2x80x9d) is completed by suitably combining a semiconductor material containing mainly silicon, an inorganic insulating material containing silicon, and an organic insulating material represented by polyimide, acrylic, or the like. Since an external quantum efficiency of the organic light emitting element still does not reach 50%, a large number of injected carriers are converted into heat and thus the light emitting element generates is heated. As a result, thermal stress is applied to the pixel portion and acts on respective layers composing the pixel portion. If the stress is large, a crack is caused.
With respect to the light emitting device using the organic light emitting element composed of a combination of an insulator, a semiconductor, a conductor, an organic compound, and the like, the interaction between internal stresses in respective films and thermal stress produced by heating cannot be neglected.
The present invention is a technique for solving such problems, and it is an object of the invention to provide measures for solving the respective factors by considering two aspects, such as an impurity factor and a structural factor, as causing the deterioration of the organic light emitting device. In addition, it is an object of the present invention to provide a light emitting device in which the deterioration of the organic light emitting element is suppressed and which has high reliability.
According to the present invention, in order to prevent the deterioration of the light emitting device, concentrations of moisture and oxygen, which are left in a space in which the organic light emitting element is sealed, are minimized. Simultaneously, an impurity including oxygen, such as moisture and oxygen included in the organic compound composing the organic light emitting element,-is reduced. Further, an element structure for preventing the deterioration of the organic light emitting element due to the stress is used to suppress the deterioration.
FIGS. 1A to 1C are flowcharts for explaining a method of manufacturing a light emitting device of the present invention. FIG. 1A shows a typical example thereof. A first conductive film is formed on an insulating film and a first electrode is formed as one electrode of the organic light emitting element. After that, the insulating film is etched to form a contact hole. This contact hole is provided so as to electrically connect one electrode of the organic light emitting element with an active element in the case of the active matrix drive. Thus, when a passive matrix drive light emitting device is manufactured, this contact hole is not formed.
Then, a second conductive film is formed and a second electrode which is in contact with the first electrode is formed. In the case of the active matrix drive light emitting device, a first wiring and a second wiring are formed using the second conductive film.
An insulating film is formed on or over the second electrode, the first wiring, and the second wiring so as to locate the end portions in the outside of them. This is not limited to the insulating film and a conductive film or a semiconductor film may be used. This insulating film is located on or over the second electrode, the first wiring, and the second wiring and forms a so-called canopy. Then, this is utilized as a mask when an organic compound layer and a third electrode are provided in a later process.
An evaporation method is used as a typical method of forming the organic compound layer. In an evaporation apparatus for forming an organic compound layer, a wall surface in the interior of a reaction chamber is mirror-finished by electropolishing to decrease the amount of gases emitted. Stainless steel or aluminum is used as a material of the reaction chamber. For the purpose of preventing gas emission from the inner wall, a heater is provided outside the reaction chamber to perform baking processing. Although the amount of gases emitted can be greatly decreased by the baking processing, it is preferably cooled using refrigerant at the time of evaporation. A turbo molecular pump and a dry pump are used as an evacuation system to prevent a contamination due to an oil steam. In order to remove residual moisture, a cryopump may be provided together with these pumps.
Although an evaporation source is basically of a resistance heating type, a Knudsen cell may be used. An evaporation material is loaded from a load lock type exchange chamber which is added to the reaction chamber. Thus, a possibility of exposing the reaction chamber to an atmosphere at the time of loading the evaporation material is minimized. Although the evaporation material is mainly an organic material, purification by sublimation is performed within the reaction chamber before evaporation. As another method, zone purification may be applied.
The structure of the organic compound layer is not particularly limited. It is obtained by suitably combining the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the like. Further, the third electrode is similarly formed by an evaporation method. After the formation of the organic compound layer, thermal treatment for emitting moisture and the like which are mixed during the evaporation may be performed at a pressure of 1xc3x9710xe2x88x924 Pa or less.
The first electrode and the third electrode are formed as the anode and the cathode, respectively, using applied materials. Thus, the organic light emitting element can be manufactured. Alternatively, the first electrode and the third electrode can be formed as the cathode and the anode, respectively. A transparent conductive material is used as a material for forming the anode and an indium tin compound, a zinc oxide, or the like can be used. A material including magnesium (Mg), lithium (Li), or calcium (Ca), which has a small work function, is used as a material for the cathode. An electrode made of MgAg (material obtained by mixing Mg and Ag at a ratio of Mg:Ag=10:1) is preferably used. In addition, there are an ytterbium (Yb) electrode, an MgAgAl electrode, an LiAl electrode, and an LiFAl electrode as the example thereof.
After that, a seal pattern is formed using an ultraviolet curable resin or the like to perform bonding of a sealing plate. Thus, the organic light emitting element is held in a closed space. Such a sealing process is performed in an atmosphere of an inert gas such as high purity dry nitrogen, helium, argon, krypton, or neon. As a result, the closed space is filled with the gas and thus the concentrations of moisture, oxygen, and the like in this space can be sufficiently decreased.
However, even if such a treatment is performed, moisture and oxygen in the closed space cannot be completely removed. For example, there is a possibility that the concentrations of moisture and oxygen in the closed space even after sealing are increased due to degassing from the organic compound layer, its surroundings, and a wall surface of a sealing member. As a result, even if complete sealing is performed, the deterioration of the organic light emitting element cannot be prevented.
Therefore, in the present invention, after sealing, a temperature cycle of heating and cooling is performed to promote degassing, and processing such that the gas is absorbed in the drying agent provided in the closed space is performed. The heating is performed to promote degassing. Barium oxide (BaO) is used as the drying agent. Since a reaction of BaO and moisture is an exothermic reaction as indicated in the following reaction formula (2), the reaction is further promoted by decreasing temperature to perform cooling. Thus, a temperature cycle of heating and cooling, or cooling and heating is repeated to further decrease the concentrations of moisture and oxygen in the closed space.
BaO+H2Oxe2x86x92Ba(OH)2xe2x80x83xe2x80x83Formula 2
After that, a power supply test is performed for aging the organic light emitting element. This power supply test has two purposes. One purpose is to stabilize the organic light emitting element and to detect an initial failure. The other purpose is to perform degassing processing. In the case where the organic light emitting element emits light at an intensity of 1000 cd/cm2, when this is converted into a photon, it corresponds to the amount of emission of 1016/second cm2. When it is assumed that a quantum efficiency of the organic light emitting element is 1%, a necessary current density is 100 mA/cm2. Joule heat is produced by a current flowing at this time and thus the organic light emitting element generates heat. By this heat generation, there is a case where impurities included in the organic compound layer, in particular, moisture and the like, are emitted. In order to effectively absorb moisture in the drying agent, the temperature cycle may be repeated again.
FIG. 2 is a diagram explaining the progression of a temperature change in the organic light emitting element obtained by such a manufacturing method. The organic light emitting element is formed at a room temperature (temperature increase due to evaporation is neglected here). After that, thermal treatment is performed at a temperature such that the organic compound layer is not deteriorated to perform dehydration processing or deoxygenation processing. Third electrode formation, seal pattern formation, sealing plate bonding are also performed at a room temperature. After that, the temperature cycle of heating and cooling is performed. With respect to a heating temperature, a temperature such that the organic compound layer is not deteriorated is also set as a maximum temperature. However, the entire element including the sealing plate is heated for dehydration processing. Therefore, it is desirable that the heating temperature is 60xc2x0 C. or higher, preferably, 80xc2x0 C. or higher. Cooling is performed to promote the reaction of BaO and cooling at 0xc2x0 C. or lower, preferably at a temperature lower than xe2x88x92100xc2x0 C. is desirable. The order of heating and cooling may be changed and desirably repeated plural times. Thus, a dew point in the close space is set to be xe2x88x9250xc2x0 C. or lower, preferably, xe2x88x9280xc2x0 C. or lower. Also in this case, one purpose of the power supply test is to heat the organic light emitting element during light emission to positively perform dehydration processing.
The reliability of the light emitting device using the organic light emitting element can be improved by such processing. Also, as shown in FIG. 1B, processes from insulating film formation to third electrode formation are performed as in the above case and then a diamond like carbon (DLC) film having high gas barrier characteristic may be formed to cover the organic light emitting element. The DLC film has Sp3 bonding as bonding between carbons in a short range order but becomes an amorphous structure in macro. With respect to the composition of the DLC film, carbon is 95 to 70 atoms % and hydrogen is 0.1 to 30 atoms % and thus the DLC film is very hard and has a superior insulation characteristic. Such a DLC film has a characteristic such that permeability of gases such as steam and oxygen is low. Also, it is known that the DLC film has a hardness of 15 to 25 GPa from measurement by a microhardness meter. As a result, entering of moisture from the outside is blocked and the deterioration can be prevented. Thus, the temperature cycle processing after that is not particularly performed.
With respect to processes as shown in FIG. 1C, processes from insulating film formation to sealing plate bonding are performed in the same manner as in the case of FIG. 1A. However, thermal treatment is not performed during these processes. After that, the temperature cycle processing and the power supply test are performed. The processes indicated here are one example and the present invention is not limited to only the processes indicated here. The present invention is characterized in that the dew point of gas in the sealed closed space is decreased by the thermal processing for the organic compound layer or the temperature cycle processing for the organic light emitting device, whereby the reliability of the organic light emitting device is improved.
According to a structure of the present invention, there is provided a light emitting device comprising: a first electrode located on a first insulating film; a second electrode which is in contact with the first electrode; a second insulating film formed on the second electrode; an organic compound layer located on the first electrode; and a third electrode located on the organic compound layer, in which end portions of the second insulating film are provided outside end portions of the second electrode and are formed at positions not overlapping with end portions of the organic compound layer.
Further, according to another structure of the present invention, there is provided a light emitting device comprising: a first electrode located on a first insulating film; a second electrode which is in contact with end portions of the first electrode; a second insulating film which is provided on the second electrode and its end portions are located outside the second electrode; an organic compound layer located on the first electrode; and a third electrode located on the organic compound layer, in which end portions of the organic compound layer are formed at positions not overlapping with end portions of the second insulating film.
Further, according to another structure of the present invention, there is provided a light emitting device comprising: a first electrode provided between a first wiring and a second wiring; a second electrode connected with the first electrode; an organic compound layer located on the first electrode; and a third electrode located on the organic compound layer, in which the organic compound layer and the third electrode are provided inside the first wiring and the second wiring.
Further, according to another structure of the present invention, there is provided a light emitting device comprising: a first wiring formed on a first insulating film; a second insulating film provided on the first wiring; a second wiring; a third insulating film provided on the second wiring; a first electrode provided between the first wiring and the second wiring; a second electrode connected with the first wiring; an organic compound layer located on the first electrode; and a third electrode located on the organic compound layer, in which end portions of the second insulating film are provided outside the first wiring, end portions of the third insulating film are provided outside the second wiring, and the organic compound layer and the third electrode are provided inside the first wiring and the second wiring.
Further, according to another structure of the present invention, there is provided a light emitting device comprising; a first organic compound layer provided on the first electrode; a second electrode provided on the first organic compound layer; a third electrode provided between the second wiring and a third wiring; a second organic compound layer provided on the third electrode; and a fourth electrode provided on the second organic compound layer, in which the second electrode is connected with the fourth electrode in an outer edge portion of the pixel portion.
Further, according to anther structure of the present invention, there is provided a light emitting device in which a pixel portion is formed on a first insulating film, comprising: a first electrode provided between a first wiring and a second wiring; a first organic compound layer provided on the first electrode; a second electrode provided on the first organic compound layer; a first insulating film and a second insulating film which are provided on the first wiring and the second wiring, respectively, such that end portions of the first insulating film and the second insulating film extend beyond side portions of the first wiring and the second wiring; a third electrode provided between the second wiring and a third wiring; a second organic compound layer provided on the third electrode; a fourth electrode provided on the second organic compound layer; and a third insulating film and a fourth insulating film which are provided on the third wiring and a fourth wiring, respectively, such that end portions of the third insulating film and the fourth insulating film extend beyond side portions of the third wiring and the fourth wiring, in which the first organic compound layer is provided so as not to overlap with the end portions of the first insulating film and the second insulating film and the second electrode is connected with the fourth electrode in an outer edge portion of the pixel portion.
According to anther structure of the present invention, there is provided a light emitting device in which a pixel portion is formed on a first insulating film, the pixel portion including: a first electrode provided between a first wiring and a second wiring; a first organic compound layer provided on the first electrode; a second electrode provided on the first organic compound layer; a third electrode provided between the second wiring and a third wiring; a second organic compound layer provided on the third electrode; and a fourth electrode provided on the second organic compound layer, in which the pixel portion is provided in a closed space produced by a sealing member and a concentration of oxygen and moisture in the closed space is 2 ppm or lower.
According to anther structure of the present invention, there is provided a light emitting device in which a pixel portion is formed on a first insulating film, the pixel portion including: a first electrode provided between a first wiring and a second wiring; a first organic compound layer provided on the first electrode; a second electrode provided on the first organic compound layer; a third electrode provided between the second wiring and a third wiring; a second organic compound layer provided on the third electrode; and a fourth electrode provided on the second organic compound layer, in which the pixel portion is provided in a closed space produced by a sealing member, the closed space is filled with at least one gas selected from the group consisting of nitrogen, helium, argon, krypton, and neon, and a concentration of oxygen and moisture in the closed space is 2 ppm or lower.
According to a pixel structure using such an organic light emitting element, since the organic compound layer is formed on the first electrode and is not in contact with other members on the first electrode, the organic compound layer is never affected by imposed stress. Thus, the organic light emitting element is prevented from being deteriorated by thermal stress due to a temperature change in surroundings or self-heating.
As described above, according to the present invention, two aspects, an impurity factor and a structural factor, are considered as the causes of deterioration of the organic light emitting device and means for solving the respective factors are provided. Hereinafter, the present invention will be described in further details through the embodiments.